The Principle of two weaknesses - one weakness or two?

One sick pawn can lose you the game, but you need two points of
attack to win. How does this add up?

Both are true. In order to win against one weakness you
need to attack the weak point, gain an advantage in space or
mobility through this, and use your mobility to force through on a
second front. Then one or other point will crack because your
opponent's pieces won't be able to cover both attacks.
Chekhover-Rudakowsky is a nice example of this.

In fact defensive play is so awful that your opponent
will probably give you an extra target out of carelessness or
frustration (Hutchings-Keene, Vogt-Anderssen).
But if not, you will have to open up another front
(Lasker-Capablanca). It may be that before going for your
primary target you should take time out to create a second weakness
for later (Hug-Barle). The key to understanding space
advantages is that you can use it to create weaknesses where none
existed (Tarrasch-Mieses).

Both Black bishops threaten to kick a hole in the White
pawn formation by ...Bxc3 or ...Bxf3.

6.Bd2 c5 7.a3 Bxc3 8.Bxc3 Ne4 9.Qc2 Nxc3 10.Qxc3
Qf6

Renewing the threats against c3/f3.

11.Rd1 Bxf3 12.Qxf3 Qxf3 13.exf3 Nc6

14.dxc5 {?}

White's f-pawns aren't pretty but Black can't get at them
at the moment. In any event, one weakness won't lose the game.
Instead of avoiding further concessions, White hands Black the
half-open b-file and a target on b6. Any 'pressure' White hopes to
gain on the d-file will vanish the moment Black plays ...Nd4.

Chess Quotes

"Many players, even of a high calibre, will assert, half jokingly and half seriously, that a difficult labour of analysis can be replaced by intuition. 'I played this move in a flash - it was obvious it couldn't be bad' is the sort of thing we often hear in a post-mortem.